Ever since the work of Charles Goodyear and Charles Mackintosh to introduce vulcanized rubber to the market to replace leather, fabric, and other materials, footwear has benefited from many advances in the art. Water and abrasion resistance have made many synthetic polymeric materials preferred for outsoles of shoes and boots. People wearing such shoes and boots in inclement weather or other rugged conditions truly appreciate the protection and warmth of modern synthetic materials used in such articles of footwear.
Conventionally, the outsole is that portion of the shoe or boot that is in direct contact with the ground. The material of the outsole must be a very durable material, especially the heel portion which in material can be the same as or different from the material of the remainder of the outsole. Synthetic rubber emerged in the mid-20th Century as a suitable outsole material, especially for heels, and preferable for the entire outsole for footwear intended for use in industrial or outdoor conditions.
More recently, polyurethane has emerged as a desirable substitute for synthetic rubber for outsoles because of superior abrasion resistance in a material that has less density than rubber. One example of advances in the polyurethane outsole art is disclosed in U.S. Pat. No. 6,759,443 (Brant et al.) for a polyurethane foam composition. Preferred abrasion resistant additives include a combination of polyethylene wax with poly(tetrafluoroethylene) (PFTE) particles.
When polyurethanes find application in outsoles, they are typically based on polyester polyols because they provide better abrasion properties than polyether polyols. However, polyester outsoles tend to hydrolyze over time especially when stored for extended periods. For that reason among others, the U.S. military requires use of polyether-based polyurethanes in their mid and insole, i.e. specifically use of any polyester-based polyurethanes is not permitted. The somewhat lower properties of polyether-based polyurethane compared to polyester-based polyurethanes have prevented the use of polyurethane in outsoles in combat boots for the U.S military to date.